Why did the angel tell Joseph to name Mary's son Jesus?

Answer: He would save his people from sin

The angel told Joseph, 'You are to give Him the name Jesus, because He will save His people from their sins.'

“But after he had pondered these things, an angel of the Lord appeared to him in a dream and said, “Joseph, son of David, do not be afraid to embrace Mary as your wife, for the One conceived in her is from the Holy Spirit.”— Matthew 1:20 (BSB)